Type printing telegraph system with means for eliminating interference



Maly 3 1, 1938. A. BAKKER :TAL 42,119,196v I TYPE PRINTING TELEGRAPH SYSEEM WITH MNS-FOR ELIMINATING INTERFERENCE Filed-Mays, 1934 z-sheets-sheet 1 /EW I 5W.'

May 31? 1938 A. BAKKER ETAL 2,119,196 TYPE PRINTING TELEGRAPH SYSTEM WITH MEANS FOR` ELIMI'NATING INTERFERENCE Filed May 9,-.1934 -2 sheets-sheet. 2`

1% :s l A 61 I i* F/6L@ F/xs Patented May 3l, 19.38

TYPE PRINTINGVTELEGRAPH SYSTEM WITH LIEAN S FOR ENCE ELIMINATING INTERFEB- Arjen Bakker, The Hague, and Hendrik Cornelis, Antonie Van Duuren, Noordwijkerhout, Netherlands, assignors to Radio Corporation of America, a corporation of Delaware Application May. s, 1934, seria: 10.724.804

In Great Britain May 9, 1933 3 Claims. v(Cl. 178-69) In operating witha type-printing telegraph system on a connection in which interference may occur, for example aradio connection, a disadvantage, as compared with systems in which 5 the actual signal is recorded, is that a wrong signal cannot be recognized as such. In systems in which the actual signal is recorded it is occasionally possible for a skilled telegrapher to recognize wrong signals.

The object of the invention is to provide a type printing telegraph system in which the unnoticed reproduction of signals or signal elements wrongly receved'may be avoided.

'Ihis object is attained by the fact that the signals are transmitted by means of double currente-that is to say the working elements of the signals are transmitted by means of Working or marking current and the resting elements by means' of resting or spacing current. Furthermore, in the receiving circuit an error-detecting device is provided whereby the presence of disturbing impulses or the dropping outcj signal elements is made recognizable.

The construction of the printing telegraph system so as to achieve the object of the invention results in 'a marked increase in traflic' trans'- an indicatingcircuit which renders the disturbed i `signals recognizable.

The invention may be employed in all known type printing telegraph systems. It has particular. advantages in two-way communication between two stations, and in systems in which, for

the purpose ofeliminating interference, each cording to the invention, employed for two-way communication between two stations.

Figures 5-9 relate to three constructional examples for systems in which the signals or signal elements are transmitted and received a plurality of` times.

In .Figure 1, the error-detecting device comprisesrtwo relays, one of which is acted upon by the working (marking) current and the other by the resting (spacing) current. of the transmitter. The tongues of these relays co-operatein such a manner that when the reception'is not disturbed i rent impulse to be annulled, the' 'tongues both lie indirectly operating the` signal element is sent out several times by the modulated on a common carrier wave.

fin the working position, or`- both in the resting erroneous by means of an additional circuit which is rendered operative in response to the occurrence of the interference.` It is also possible to prevent the actuation of the printing apparatus in response tomutilated signals. Instead of this, 'it is also possible to keep the disturbed signals away from the receiving apparatus.

At the transmitting end double current is ernployed. In the case of a radio connection for example,` a so-called double Wave transmitter or two signal frequencies are employed, which are In any case, the .working signal elements are transmitted with working current and the resting signal elements are transmitted with the resting current.-

The two sources of current, which produce the Working current and the resting current, are shown at l andv 2, and 3 is the tonguel ot the transmitting relay. This circuit arrangement of *the transmitting station is not essential for the invention.

At the receiving end, both signal currents are rreceived, and are thereupon separated from one another by lknown means such as lters fi, E. The resting current and the Working Acurrent are now fed to rectiers 6 and 1 respectively and these rectiers act upon relays 8 'and 9 respectively. The tongues of these relays are applied against. the resting contact r when the, corresponding .rectier does not receive anysignal voltage,and

against the working contact w when the rectier receives signal voltage.

Due to the double wave transmission, both relay tonguesoccup'y opposite positions when there is no disturbance in the reception. When the tongue of the relay 8 occupies the working position (w), the tongue of the relay 9 occupiesrthe by a lead I0 to the printer apparatus. Thefresting contact of the relay 8 and the working contact of the relay 9 are connected together and to an interference relay II,.while the working contact of the relay 8 and the resting contact of the relay 9 are both connected to a source of current I2. One side of the relay II and one side of the i source oi current I2 are grounded.

Cil

With thisconnection, the interference relay II receives current when the tongues of both relays 8 and 9 occupy the resting position r (interference through fading) or when they both occupy the working position w, (interference through an additional 'current impulse). In the event of undisturbed reception, on the other hand, the relay II does not receive any current.

The printing apparatus connected to the lead I0 receives working current when the relay is in the working position and the relay 9 is in the resting position. It.does not receive any current when the relay 8 is in the resting position and therelay 9 is in the working position, and t receives working current when both relays occ y the same position/(that is to say in the case oi' interference). In the last case. however, the received signal may beindicated as an error by means of the interference relay II, or in any other manner. 'I'his relay,.therefore, indicates erroneous signals with absolute certainty.

It is obvious that the interferencev relay .I I may act upon the printer in many diilerent ways. Thus, for example. ablank or another interference signal, such as a special printed sign. may be given. Furthermore, the starting of the printer may .be prevented in systems in which the code signal is followed by a locally initiated printer-actuating impulse. The practical execution dependsV upon the particular telegraph apparatus employed, and is not an essential feature of the invention.

According to a modiiication of the invention the occurrence of a wrong signal element may be rendered recognizable by the employment of means responsiveto such an-erroneous signal for intercepting the working 'or resting current as -supplied to the printer. The printer will then function aswhen spacing"instead 'of printing erroneous characters. 'Ihis i'eature'is of considerable importance when using uniform length code signals in printing telegraph systems', as will be shown more clearly hereinafter.

Fig. 2 illustrates how the relays l and 9 may be so employed that. the .disturbed signal element is kept away from the printer. As will be seen from .this figure, the printer does not receive any current when the tongues of the relays 8 and a are bonn in the' resting position or both station A has been shown in the drawings.

in the working position, because the current sources I2 and I3 Will be connected together. If, on the other hand, the received signal is undisturbed, the telegraph apparatus receives position, remains in this middle position, so that the telegraph apparatus does not receive any current.

The invention as used in two-way communication renders it possible to repeat the transmission of a certain signal by one of two stations, if this signal is not correctly received at the other station. If, therefore, a. signal is mutilated during transmission, it is repeated and if it is notl mutilated it will not be repeated.

According to the invention, this method is preferably carried out in such a manner that the reception of a mutilated signal results automatically in a warning signal being sent back, whereby the signal is repeated in the transmitting station, while the reception of an linterference-free signal results automatically in a monitoring signal being sent back, whereby the succeeding signals of a message are transmitted in regular course.

Figure 4 will serve for explanation of the foregoing. In this ligure, a constructional form of the system according to the invention is shown ldiagrammatically by way of example, it being assumed that each transmitted signal consists of ilve elements (5-unit code). The two-way or cross trailic takes place between two stations 'A and B, .which are substantially constructed in the same way. Forthe sake of simplicity, only Each station has on the left hand side a receiving device and on the right hand side a transmitting device. 'Ihe parts of the individual station are denoted by numbers, and will be distinguished from one another by the addition of the letter A or B. Thus, for example, AIB is the receiving apparatus of station A and BIB the receiving apparatus Iof station B. A

The vertical strokes under the number I represent the4 nl oments at which a distributor (or any other equivalent device) connects the receiving device'of each station with the ytransmitter, say, through a radio communication channel, the vertical strokes under the number 2 represent the monents at which another distributor connects the transmitting device with the channel of communication. 'I'he individual moments of each vertical series follow one another in the direction of the arrows shown on the drawings. l

The transmitting distributor A2 of station A runs synchronously with the receiving distributor BI of station B, and the transmitting distributor B2 of station B runs synchronously with the receiving distributor AI of station A. Both distributors in each station may be coupled together mechanically. It is immaterial whether synchronism is obtained by means of distributors moving continuously or discontinuously. In

, the present instance an installation with chronisin of the distributors in the transmitting station andthe receiving station are sent out 't by the transmitting station B through distributor segments B3 and B4, and are received bythe receiving station A on segment A5. The correction magnet `of station A is connected to a protective device A6. The latter operates in the manneralready described in the foregoing and may be so constructed, for example, that if an interference occurs while the receiving distribuf tor brush is traversing segment A5, the correction magnet of station A will not operate. Premature correction is, therefore, avoided.

The ve elements of each signal are transmitted by station B on segments B26 to B30, and

are received by A onsegments A1 to AII whence' they are transferred to the protective and signal storing device AI2. The protective device servesv to ascertain whether one or-more of the five signal elements received has been disturbed. If none of the five elements is disturbed, the interference relay AI3 remains unenergized, and the actual telegraph recording apparatus AIS receivesastarting current impulse by way of the resting contact Alt of the relay `AI3 and a con tact of the distributor AI, which latter contact is not shown. The telegraph apparatus AI then becomes operative.' receiving the signal elements stored up in AI2 and converting them into printing or recording or interpreting them in any other manner. Furthermore,`a positive (spacing) current impulse is sent through segment AIB to station B by way of the tongue AIG of the polarized relay All and is received by B on segment BIS. This,v current impulseis received by a device B20, which is also constructed as a protective device. d An interference relay B2I s connected to this device. If the spacing current impulse transmitted by A is received undisturbed, the interference relay B2I` is not energized, and consequently the transmitting apparatus B22 receives a starting current impulse by Way of a contact (not shown) .0f the corresponding distributor from the protective device B20 by way of the resting contact B23 of the interference relay B2i. The trans.- mitting apparatus B22 is thereby rendered operative. The spacing contacts of the ve transmitting tongues of this apparatus are connected at B24 ,to the positive-terminal of a source of current, and the marking contacts of the tongues are connected through the spacing contact of the interference relay B2I to the negative terminal of a source of current. By the starting of the transmitting apparatus, the transmitting tongues are connected to voltages which correspond to the next signal, so that this signal is now transmitted at the moments B26 to B30.y

In the station A, this signal is again received at the moments A'l to AII. The process described in the foregoing is Vthus repeatedand the second signal is also printed and so on.

If, in transmitting the first signal one or more elements of this signal are received disturbed in" the station A at the moments A1 ,to `Alljthe interference relay AI3 is energized.l

` This relay, like the interference relays 2| and 35, on` being energized remains in the energized condition for one complete revolution of the distributor. At the end of this revolution it is either de-energized or re-energized according to whether ,the current impulses received at this moment c are undisturbed or disturbed.

When the relay AI3 `is energized the contact AM is broken, so thatthe receiving apparatus AI5 does not receive any starting current im pulse, and consequently does not record any signal.` At the same time, through the energizing of the relay A23, the relay Allis given a negative current impulse, whereby a negative current impulse (working current impulse) is y transmitted from the station A to the station B on segment AIB by way of the tongue AIB of the said relayAI'I.

This marking current impulse is received on 'segment BIS by the protectivedevice B20 and signal, to repeat the signal which has already once been transmitted.

As described in the foregoing, this request is received at the station 1B` on segment BIS, there being a possibility that, due to an interference, the warning signal transmitted by station A is not received correctly at station B. Since then it is` not possible to ascertain whether a repeat o has been requested or not, in this case for the sake of certainty no fresh signal is transmitted and also the old signal is not repeated. This may be effected for example, as follows:

If an interference is received at the moment BI9, the interference relay B2i is energized. The starting circuit of the transmitting apparatus B22 is interrupted at B23, The tongue B25 of the interference relay B2i connects the positive terminal of a source of current to the working contactsof the five transmitting tongues, so that on segments B26 -to B30, five spacing current impulses are transmitted. The receiving apparatusfof station A does not respond to these five spacing impulses, since the latter correspond to the resting condition of the transmitter.

The receiving device of station A, however,

' must ascertain whether the station B at segment BI9 has or has not reacted to the last positive or negative warning current impulse transmitted by A, because only in thi-slcase is the station A in a position to request, if necessary, the repetition of the last signal or the transmissionv of the next signal. Y

Station A could ascertain the disturbed Vreception of the warning impulse by the fact that five f spacing impulses are received from the station B. In view of ,the possibility that these impulses may be disturbed, itis preferable to transmit a special signal from station B, in addition' to the five spacing impulses. This signal is preferably transmitted on segment 3| and preceding the five spacing impulses. Normally, that is to say, if the warning signal is received correctly by B20, thespecial signal consists of a negative current impulse (working. current impulse), which is transmitted by way of the resting. contact of the tongue B25. In station A, this signal is received through segment A32 by a protective device A33 which shows in known manner whether .the signal received consists of a. resting current impulse, a working current impulse, or an interference.

By the reception by A33 of a (negative)l working current impulse or an interference, the circuit oi'the relay All is connected to earth by a contact (not showniof the distributor by way of a contact (likewise not shown) within the device A33, and through the resting contact A34 of the relay A35. The.contact within the device A33 interrupts the circuit, however. if the special signal consists of a (positive) resting current impulse. Furthermore the circuit is likewise broken,l if the relay A35 is energized, which only takes piace in the event of undisturbed reception of the tive resting units by the device All.

By the use of this circuit, the, relay All can never be iniluenced by the fact that the signal received by Ail is or is not disturbed, if the special signal received by A33 consists of a positive or resting .current impulse. In .this case, the circuit of the relay A II is already interrupted within the device A33 itself so thatthe tongue A16 remains in the position which it had attained after recepvthe moment AIB retransmits the same warning signal in the form of a resting or working cur,v rent impulse by way of the tongue AIS. If this current impulse is now received in B by B20 undisturbed, the repetition of the previous signal or the transmission of a fresh signal can take place. On reception in A of a negative -current impulse or a disturbance, however, the signal to be received at the moments A1 to AI I is stored up and any disturbance of this signal is reported by the relay AI3.

The probability of the warning signals and the other special signals being transmitted undisturbed may, of course, be increased in different ways, for example, by multiple transmission of these signals, or by lengthening or shortening them. It is also possible, by such means, to reduce the possibility oi.' interference of the five elements of each signal. The time distribution of the above-mentioned contact moments shown in the ligure, the proportion of their length and the number of these contact moments are therefore not essential. They may be varied without departing from the principle of the invention.

It is obvious that a certain time isnecessary in order'to transmit by means of the transmitting apparatus of the receiving station a, warning signal to the transmitting station. 'I'his signal can only be transmitted when the ve elements of the transmitted signal have been received and examined. Some time elapses, therefore, before th'e transmitting apparatus of the transmitting station can start again. The time of waiting between the successive transmissions of the same or different signals may if necessary be lled in with the signals of another telegraphD system which employs the same transmission channel. This may be effected by means of-contact members, which bring about in known manner a suitable time distribution, so that the individual telegraph systems can employ th'e transmission channel successively.

It isfurthermore obvious that the connections as shown in Figure 4 may also be constructed `otherwise within the scope of the invention.

Thus, for example, the devices 3, I2, 20 and 33 are not connnedto a definite form of construction. This already follows from the discussion of the protective devices shown in Figures 1, 2

and 3. It is also not necessary to employ special interference relays i 3, 2|, 35. The circuits which are controlled by these relays may also be opened or closed by the protective devices themselves.

A further possible means of combating interference in a type printing telegraph system according to the invention resides in the application of the known principle namely, that interference may be eliminated up to a certain extent by repeated transmission and reception of all signal elements and signals.-

The repeating systems known for this pzrpose are generally only capable of combating interference of a certain character, that is to say, either interference due to additional current impulses, or interference due to the absence of current impulses.

For telegraph systems in which both types of vinterference may occur (in the case of radio telegraphy, for example, fading and atmospherics interference), so that resting signals and working signals may be incorrectly received, efforts have been made to discover means in order either to give the fading interference the character of atmospheric interference (that is to say, of additional current impulses) or to give the atmospheric interference the character of fading interference (that is to say of absent currnt impulses); The said means vare provided in the receiving installation in front of the repeating system and the actual receiving apparatus, and result in the receiving apparatus being supplied incorrectly either with working signals only or ywith resting signals only. In such systems, the repeated reception of the working current element or resting current element results in the formation of a working current signal or a resting current signal.

This property of the known systems immediately shows the great disadvantage of suchsystems, namely, that the repeated reception of working or resting current elements, which are not caused by a repeatedly transmitted signal but by interference, can lead to the formation of a signal. This signal must-then be wrong, however, and the mistake will not be noticed.

If, on the other hand, the principle of repetition is employed in combination with the present invention the undisturbed reception of a` single transmission is sumcient in order to render possible the reproduction of the correct signal in the actual receiving apparatus because each interference is. indicated with absolute certainty. If none of the transmissions is received undisturbed this is also rendered recognizable with certainty in the receiving apparatus. The unnoticed reproduction of a wrong signal is therefore impossible. Several constructional forms will be de- 'further to the telegraph apparatus, so that the latter is set in operation. If, however, none of the signals passed on by the protective device is undisturbed, an interference signal is supplied in any manner tothe telegraph apparatus, or the operation of thesaid apparatus is prevented.

The testing device must satisfy the condition of being able to ascertain and store the diierence .75

2. Relay circuits which are adapted to receive' and re-transmit the diie'rence between a positive current impulse, a negative`current impulse and zero current impulse, or between current impulses of different intensities', or between direct current and alternating current.

3. Relays in combination with mechanical devices for receiving. storing and retransmitting the three different units.

In Figure 5I is shown a testing device employing a distributor or any other equivalent device which is, driven synchronously, in rhythm with the repeated transmission," with the repeating device of the transmitter, which supplies successively a single contact.

to a condenser the quantities of electricity, given by the protective device during the repeated transmission of each signal and which subsequently employs the charge of thissaid condenser for actuating the receiving apparatus.

It has been assumed in the iigure that`each signal element is received three timesand hence can be tested three times.

The current impulses which are given by a protective device, not shown (for example according to Figure 2) and which may compriseapositive current, a negative current or zero current in order to give a resting signal, a working signal, or an interference signal, are supplied to the lead 25. 2|, 22 and 2,3 `are contacts of a distributor (contact disc, contact drum. or the like), which are closed just at the moments at which the repeated signal elements are received in the lead 25. It depends upon the construction of the distributor whether the contacts 2|, 22 and 23 are separated from one another or arecombined to form The construction of the distributor is of course optional, the only condition being that it shall operate synchronously with the `repeating system of the transmitter.-

When the contact 2| is closed, the rst current impulse givenby the protective device has the opportunity of charging or not charging a condenser 26. The contact 22 doesthe same for the second current impulse and the contact 23 for the third current impulse. Sincethevthree current impulses belong to the same signal element, the condenser 26 may be charged only in a certain direction either positively or negatively, or. may

remain uncharged. Current impulses of opposite directions are not possible in the same signal element, since the protective device either passes on the received signal correctly or not at all, and a wrong transmission is impossible.

'After the lapse of the third repetition, the condenser is hence charged certainly in the correct direction, ii an undisturbed signal element has been received at least'once by' the protective device. '-If this is not the case, the condenser is not charged.

, After the condenser has functioned to receive three times a correct current impulse the dis.

the polarized relay'21 is applied to the resting y relay.

contact orv the working contact, arestingeor working current impulse being passed on to the telegraph apparatus by the energizing of the relay 28. If the condenser 26 has no charge, the nonpolarized relay28 isnot energized. An interference signall can be then transmitted to the telegraph apparatus by the resting contact of The non-polarized relay hence serves to distinguish current and zero current from Aone another, and the object of the polarized relay is to transferA the current Ldirection (Working or resting current) to the receiving apparatus. In other respects, it is clear that the circuit in which the condenser 26 is discharged may be constructed in different ways. This construction depends, for'example, upon the nature of the telegraph apparatus connected thereto.

When, after the repeated transmission of a definite signalelement, the condenser 26 has performed its action, it is in a position to receive the current impulses -oi a following signal element. limited to one signal element merely for the sake of simplicity, for example tothe firstsignal element of a signal. comprising five elements.

ber of relay chains are employed. It has been assumed that each signal element is transmitted twice. 'I'he relay chains are closed one after the other by a distributor or an equivalent device, driven synchronously with the repeating device of the transmitting end, at moments in which the individual current impulses or zero current impulses, given by the protective device during the repeated transmission of the same signal ele-L' ment, arrive lat the testing device. The relay this The explanation given above has been Y chains are constructed 'in such a manner thatv the two relays of the protective device are situl ated a polarized relay and a non-polarized relay.

With this circuit, asin Figure 2, on the reception' o'f an undisturbed signal element, a resting current `,or a working current is sent into the lead 35, and on the reception. of a disturbed signal element, no current occursin the said lead.

The relay chains of the testing device each comprise a non-polarized relayfll witha holding contact 36, and in series therewith a polarized relay Il.l The contact 3| of the distributor is closed at the moment at which the protective device operates on the iirst reception of thetwice transmitted signal element. If the reception is undisturbed, the protective device gives rise to a working current or aresting current and the nonpolarized relay is energized .and is kept energized bymeans of its holding contact 35. 'The polarized relay 42 applies its tongue.- in accordance with the direction of the received current impulse, 'either to the resting contact or to the working contact, so that this current impulse is passed on by way of the contact' 31 of the non-polarized relay 4I to the second relay chain, whereby this second relay chain which was originally connected to the lead 35 by the contact 39 is disconnected from the protective device.

` At the moment of the reception of the second transmission, the contact 32 oi the distributor is closed, the non-polarized relay 43 is then ener-v gized and is maintained energized by way of the contact 38. while, by means of the polarized relay 44, a current impulse is supplied to the teleiirst relay chain will be ready to receive a new current impulse, whilst the telegraph apparatus graph apparatus. The contacts 33 and 34 of the distributor have the object of short-circuiting the holding winding `of the relays 4| and 43 and of returning the circuit into the original state, after the respective relay chains have completed their action.

If an interference occurs in the rst transmission of the signal element, the protective device does not transmit any current impulse. In-this case, the non-polarized relay 4| is not energized, the result of which is that the second relay chain remains connected to the lead 35. On the second reception of the repeated signal element, the second relay chain then comes into action. If this `reception is undisturbed, the relays 43 and 44 are is still being fed by the second relaycircuit.

Figure 'l shows a third construction of the testing device. This operates substantially in a mel chanical manner.

At the transmitting end of the telegraph system, a perforated strip 51, on which the signals to be transmitted are iixed, is passed through three automatic transmitters Il, 52, B3 in succession. These transmitters are so connected to a distributor 59 that they' are connected in succession to the transmission channel.

The perforated strip runsthrough the three transmitters and the elements of each signal are transmitted automatically three times. The time intervals between the successive transmissions are quite optional. I

At the receiving end are three so-called reperi'orators-M, 55, I6. 'Ihese are located behind a distributor 0|, which followsthe protectivev device Gl and is driven synchronously with the distributor I9 of the transmitting station. 'I'he receiving distributor connects the three reperforators to the transmission channel at the moments at which the current impulses originating. from the transmitting station set the protective device 80 into operation.

A strip 50, the speed of which 'is` equal to that of the perforated strip 51 `of the transmitting station, is passed through the three reperforators.

A givensignal is transmitted on the first reception to the reperforator 54 and this reperforator nal element or, if one or more of the si elements are received disturbedydoesnotproduce periorations for these elements. l

lproduces a perforationin the strip for each sig- In order that the reperforators can show the difference between the three different units (positive current impulse, negative current impulse and no current impulses), the perforations caused for example by a positive or resting current impulse are oifset in the direction of the strip relatively to the perforations caused by a negative or working current impulse. The relative position is shown in Figure 8. This figure lrelates to the case in which each signal comprises five units. In the transverse direction of the strip there is room for ilve perforations. The perforations for the resting units are not situated, however, on the same transverse line as those on the working units. It has furthermore been assumed that the signal which should be received by the reperforator l64 consists of three resting units and two working units.

At the moment at which the repeatedly transmitted signal is received for the second time, the

distributor switches in the second reperi'orator 55. At this moment, the strip 58 has been advanced until the perforatlons caused by 54 on the iirst reception are situatedexactly opposite the perforating members of the reperforator 55. If now on the first reception, no perforation was produced at certain points of the strip 58, due to interference of one or more of the ilve signal elements, the reperforator can correct Ithis fault, unless an interference again occurs just at this moment. The same applies for the third reperforator 56. which comes into operation on the third reception of the transmitted signal.

'I'his testing device also operates in such a manner that the receiving apparatus is set into operation when the protective' device passes on at least once a signal (or signal element) lwhich has not been mutilated byainterference.

When, due to the occurrence of interference, none of the three.reperforators is successful in producing a correct perforation. there is in the strip a. non-perforated place which is shown in Figure 9 and which renders recognizable the wrong reception with absolute certainty.

After the strip has left the last reperforator it is passed for example to an apparatuswhich converts the, perforations to type printing. lI'his type printing apparatus may be so constructed that in the absence of a .perforation it produces an interference signal. l

In addition tothe provisions on the lines set forth in the foregoing, of a protective device and a testing device, the invention may also be employed in telegraph systems with repeated transmission of all signals, by combining a protective f device with two known repeating systems, one for the working current and the other for the resting current.

1.'In a. telegraph system of the characterdescribed, a receiver for receiving working and resting current impulses, and a protective device associated with said receiver for protecting the reassociated with said receiver for protecting the receiver against unnoticed reproduction of false 2,119,196 signal elements, said protecting device compris- A l ing two relays, the armatures oi' which are shiftable to working and resting positions, yone of said `relays being controlled by working current and the other being controlled by resting current, said relays nbeing operable in the event of undisturbed reception so that the amature of one relay is in working position andthe armature of the other in resting position, and so that in the event of interference both relay armatures are in the same position, and means operable upon the armature of one relay being in the working position and that of the other relay being in the resting position for passing the signal elements to the receiving apparatus and when both armatures are in the same position for rendering th interference recognizable.

.3.' In a telegraph system of. the character ,de scribed, a receiver for receiving working and resting current impulses, and a protective device associated with said receiver for protecting the receiver. against unnoticed reproduction of false signalelements, said protecting device comprising a relay having two windings and an armature movable to working and resting positions,`

one or said windings being controlledby working current 'and the other winding being controlled by resting current, said windings being.

opposed so that in the event of undisturbed reception the armature isV moved to either the working or resting position and in the event of interference the armature will be held in neutral position, and means operable upon the shifting of the armature to either the working or resting position for passing the signals to the receiver.

ARJEN BAKKMv HENDRIK CORNELIS ANTON'IE VAN DUUREN. 

